The present invention relates to the flow control apparatus, especially those for use in fluid flow catheters of the type found in blood pressure and dynamic measurement systems. In such systems, a catheter commonly is inserted into a patient's vessel to a location where measurements are desired. The catheter is maintained full of a neutral, sterile solution, which actually flows into the patient at a very low rate of, say, 3 ml/hr. This low flow is required to keep the catheter open, or patent, throughout its length, by preventing the ingress of blood constituents at the in-dwelling end of the catheter. By monitoring changes in the liquid pressure in the catheter, a variety of useful data can be obtained regarding the blood pressure, flow, etc. at the in-dwelling end.
When such catheters are prepared for use, they must be flushed completely out of all air which could be harmful to the patient. Since the normal small flow rate would require such a long time for filling and flushing operations, prior art devices have made provisions for providing for much higher flow rates (rapid or fast flush mode) for use during the initial filling and flushing of the catheter and for use to momentarily allow a higher flow rate into the patient to clear out any debris which may have collected at the in-dwelling end of the catheter. U.S. Pat. No. 3,675,891 issued July 12, 1972 as well as copending application entitled Improved Catheter Flushing Apparatus, Ser. No. 06/52,019, assigned to the assignee of the instant application, disclosed such catheter flushing apparatus. These prior art devices in operation under the fast flush mode of operation, however, have suffered from the defect of possible production of air bubbles in the drip chamber of the measurement system if the operator is not careful. These bubbles which can enter the patient line pose a danger to the patient and grossly degrade the dynamic performance of this measuring system. The bubbles are generated by the Venturi effect of this high velocity jet of liquid flowing under rapid flush mode.